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Plug leak in BuildTupleHashTable by creating ExprContext in correct context.

Inbf6c614 I added a expr context to evaluate the groupingexpression. Unfortunately the code I added initialized them while inthe calling context, rather the table context. Additionally, I usedCreateExprContext() rather than CreateStandaloneExprContext(), whichcreates the econtext in the estate's query context.Fix that by using CreateStandaloneExprContext when in the table'stablecxt. As we rely on the memory being freed by a memory contextreset that means that the econtext's shutdown callbacks aren't beingcalled, but that seems ok as the expressions are tightly controlleddue to ExecBuildGroupingEqual().Bug: #15592Reported-By: Dmitry MarakasovAuthor: Andres FreundDiscussion:https://postgr.es/m/20190114222838.h6r3fuyxjxkykf6t@alap3.anarazel.deBackpatch: 11, where I broke this inbf6c614

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src/backend/executor
- execGrouping.c

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`‎src/backend/executor/execGrouping.c`

Lines changed: 9 additions & 3 deletions

Original file line number	Diff line number	Diff line change
`@@ -208,11 +208,17 @@ BuildTupleHashTable(PlanState *parent,`
`208`	`208`	`hashtable->tab_eq_func=ExecBuildGroupingEqual(inputDesc,inputDesc,`
`209`	`209`	`numCols,`
`210`	`210`	`keyColIdx,eqfuncoids,`
`211`		`-parent);`
	`211`	`+NULL);`
`212`	`212`
`213`		`-MemoryContextSwitchTo(oldcontext);`
	`213`	`+/*`
	`214`	`+ * While not pretty, it's ok to not shut down this context, but instead`
	`215`	`+ * rely on the containing memory context being reset, as`
	`216`	`+ * ExecBuildGroupingEqual() only builds a very simple expression calling`
	`217`	`+ * functions (i.e. nothing that'd employ RegisterExprContextCallback()).`
	`218`	`+ */`
	`219`	`+hashtable->exprcontext=CreateStandaloneExprContext();`
`214`	`220`
`215`		`-hashtable->exprcontext=CreateExprContext(parent->state);`
	`221`	`+MemoryContextSwitchTo(oldcontext);`
`216`	`222`
`217`	`223`	`returnhashtable;`
`218`	`224`	`}`

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Commit9cf37a5

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`‎src/backend/executor/execGrouping.c`

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